With the introduction of IEEE 802.3af standard, the number of inline powerable devices (PDs) having varying power consumption levels is expected to increase. Similarly, there is power-sourcing equipment (PSE) that supports the different levels of power that it can provide. For example, Cisco Systems, Inc. produces inline power cards that support 6.3 W devices. For backward compatibility, there are IEEE PDs that utilize the Cisco Discovery Protocol (CDP). Thus, there are combinations of PDs and PSEs that need some negotiation to make sure that the PDs don't oversubscribe the PSE. For example, it's required that a 15 W PD doesn't consume more than 6.3 W on old inline power cards.
Currently, IEEE classifications exist to denote ranges of power consumption of PDs. However, these classifications are very coarse. For example, IEEE Class 3 devices consume anywhere from 7 W to 15.4 W. This means that an IEEE Class 3 device that needs only 7 W actually requires 15.4 W to be allocated by the PSE. Power management by the PSE's power manager is therefore inefficient, and there is a need for power negotiation between PDs and PSEs that overcomes this problem.